For Enhanced Geothermal Systems (EGS) controlled by pre-existing fracture networks,
it is necessary to explore the reservoir by borehole imagery techniques during the
phase of drilling reconnaissance. As pre-existing fractures represent the permeable
pathways, such techniques permit to get at the end of the drilling a consistent
fracture characterisation in terms of location, geometry, thickness, aperture and
alteration. The main borehole imagery tools, which are based on acoustic or
electrical methods, provides an oriented mapping of the borehole wall. In this
paper, the contribution of borehole imagery to EGS reservoir is based on the
exhaustive database of the deep Soultz-sous-Forêts wells which were systematically
investigated by means of images logs.
On these high resolution images, different structures, detected due to their own
geophysical contrast, could be interpreted such as pre-existing fractures, fracture
zones, alteration zones or locally lithological variations. Structural analysis of
the borehole image logs permits to reconstruct the natural fracture network in 3D at
different scale (small-scale fractures, fracture zones) Induced fractures could also
be interpreted and measured on the borehole image logs. Their relationship with the
present-day stress field could be derived and computed. Comparison between the main
fracture sets, or the main fracture zones, and the present-day stress field brings
lot of information for understanding fluid circulation in the EGS fractured
reservoir.
However, despite of the high resolution of the tools, all fractures present in the
rock mass are not systematically visible on the image logs and about 20% of
fractures are only detected properly in crystalline rocks. Moreover, lot of
fractures are not clearly visible on the images or are not ideal 3D planes. Thus,
the measurements of their orientation could be difficult to obtain and extrapolate
in 3D and could bring some biases which have to taken into account. Similarly, there
is another limit by using borehole image logs to characterize the fracture network
which is related to the absence of vision beyond the drill wall of the fracture
extension.
In conclusion, the borehole imageries constitute reliable tools to detect fractures,
which are natural or induced, fracture zones and altered zones. These methods permit
to obtain high quality fracture datasets of a geothermal EGS reservoir and
contribute to a better understanding of the fluid circulation through a deep seated
fractured rocks.
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